1. Technical Field
The present invention relates to an electrooptic device and an electronic device.
2. Related Art
Generally, liquid crystal devices include a liquid crystal panel and a back light which is a lighting system provided on the back of the liquid crystal panel. A liquid crystal device of a double-sided display type which has two liquid crystal panels, a main panel and a sub panel, on both sides of the back light is proposed as a display of a mobile phone (for example, refer to JP-A-2003-207801). This type of liquid crystal device displays an image by illuminating a liquid crystal panel by a back light from the back to obtain transmission light from the front. Therefore, it is sensitive to the brightness of surrounding environment, and the visibility of an image may change a lot depending on the environment. Thus, in order to obtain an appropriate display irrespective of surrounding brightness, a liquid crystal device having a photosensor for measuring external brightness (ambient light), and which controls the display state of an image on the basis of the measurement is proposed (for example, refer to JP-A-2004-94098).
Known methods for controlling the display state of an image include a method of controlling the luminance of a back light. However, since only two states, “dark” and “bright” can be sensed by the known control method, it has been insufficient to control the back light.
For example, outdoors in the daytime or indoors in which a fluorescent light is off, it is always determined to be “bright”. Accordingly, the back light was too bright or too dark depending on the place. Moreover insufficient control of the back light affects the endurance of a battery for driving the back light. Thus, controlling the luminance of the back light in two or more levels according to the luminance of ambient light is examined.
Human visibility changes more sensitively in a dark environment than in a bright environment. Therefore, fine control of the back light in an indoor environment which is illuminated by a fluorescent light is effective in improving the visibility and reducing power consumption. In an indoor environment illuminated by a fluorescent light, the luminance of ambient light varies depending on the distance from the fluorescent light. For example, light detected in an outdoor environment is uniform because reflected sun light is sensed. Accordingly, when the luminance of ambient light is measured by a photosensor, the brightness of environment that human senses, i.e., an average luminance of ambient light can be measured. However, in an indoor environment illuminated by a fluorescent light, the photosensor detects light from the fluorescent light directly. Therefore, the measured luminance of ambient light varies greatly with the distance from the fluorescent light. Therefore, even if the photosensor measures the luminance of ambient light as it is, the photosensor cannot measure an average brightness that human senses. Thus, if disagreement arises between the luminance of ambient light measured by the photosensor and the brightness of the environment that human senses, the back light may be too bright or too dark depending on the place.
Thus, in controlling the luminance of the back light, it is important to measure correctly the brightness of environment used as the reference for the control. The brightness of environment that human senses is average environmental brightness. Therefore, in measuring environmental brightness by a photosensor, it is necessary to calculate the average value of the data of ambient light obtained. However, the data of ambient light varies widely. Therefore, if the average value of all the data is calculated, disagreement may be produced between the average value and the brightness that human senses. For example, the measured luminance of ambient light increases discontinuously just under a fluorescent light. Therefore, if the data of ambient light measured at such a place is included, the average value of the data will be increased, and disagreement will be produced between the average value and the brightness that human senses. Therefore, when the user moves in a passage in which a fluorescent light is installed, the luminance of the back light becomes so strong near the fluorescent light that the back light may sometimes blink during the movement.